Special heat treating method of steels



United States Patent .Ofiice 3,288,657 Ratented Nov. 29, 1966 3,288,657 SPECIAL HEAT TREATING METHGD F STEELS Sadayoshi Morita, Makoto Sato, Mitsuo Furuya, Yoshinori Yamaguchi, and Teruo Umeda, Kitakyushu, Fukuoka Prefecture, Japan, assignors to Yawata Iron & Steel Co. Ltd., Tokyo, Japan No Drawing. Filed Aug. 5, 1963, Ser. No. 300,041 Claims priority, application Japan, Aug. 8, H62, 37/34,235 Claims. (Ci. 148-143) This invention relates to a heat treating method of steels, particularly a heat treating method for improving the notch-toughness of carbon steel and low alloy steel consisting mainly of ferritic and pearlitic structures It is well known that the notch-toughness of carbon steel and low alloy steel depends upon the grain size of these steels and it will be improved by refining the grains thereof. The present invention relates, therefore, to a method of grain-refining of steels, that is, a method for obtaining the fine-grained carbon steel and low alloy steel.

Generally, the notch-toughness of carbon steel and low alloy steel may be improved by subjecting them to the heat treatment such as quenching and tempering. This is due to the crystal grains of the ferritic structure being refined and bainite and tempered martensite being produced by the quenching and tempering treatments.

However, in the ordinary hardening and tempering treatments, wherein the heating rate is slow and the holding time in the hardening treatment is long, satisfactory improvement of the notch-toughness cannot be obtained. This since the ferritic and pearlitic grains are still relatively coarse, though a little finer than those of steels not subjected to the heat treatment. If the heating rate is increased and the holding time is shortened in the ordinary heat treatment, when a steel is heated up to the temperature above the A0 transformation point, the ferritic grains will be made fine and the transition temperature after the tempering treatment will be reduced. In this manner the notch-toughness of steel may be comparatively improved. However, in this case, if the heating temperature is between the A0 and AC3 transformation points, the steel will suffer a deterioration in the strength which is considered to be caused by remaining ferritic structure. When the steel is heated up to the temperature above the A0 transformation point, the atisfactory notchtoughness may still not be achieved, as in the ordinary hardening treatment of long holding time, unless it is quenched before the growth of the austenitic grains begins, that is, in the state wherein there are still many ferrite-forming nuclei.

The present invention provides a special heat treating method, whereby such high notch-toughness of the steel as could not be obtained by any conventional method is obtained without impairing the strength of the steel.

The principal object of the present invention is to obtain such high notch-toughness as is not obtained by any conventional hardening and tempering method through a novel method in which a steel is first heated to the temperature above the A0 transformation point for quenching and thereafter reheated at least once to the temperature between A0 to A0 transformation points for quenching and then tempered.

Another object of the present invention is to obtain a high notch-toughness ot" the steel without impairing the strength thereof.

The present invention shall be detailed in the following.

The discovery of this new heat treating method as disclosed in this present application is based on the fact observed by the inventors in welding a hardened and tempered steel. In the zone affected by welding heat the part of said tempered steel, where the grains of steel are refined, shows substantially no reduction in the hardness and also the strength and further the grains in this part are much finer than those of the base metal. On the basis of the aforesaid fact various experiments were made by the inventors on the test steels mainly consisting of ferritic and pearlitic structures by subjecting them to the repeated heat treatments to obtain the same effect as by the weld-heating. As results of these experiments the inventors ascertained that by repeating the heat treatments, that is, by heating the test steels to the temperature above the A0 transformation point for quenching (the first hardening) and subsequently reheating said test steels to the temperature in the transformation range of A0 and Ac for quenching (the second hardening) and then tempering the samea much better notch-toughness of the steel may be obtained without impairing the strength. This is an improvement over carrying out the ordinary hardening and tempering treatments with a rapid heating rate and short holding time, that is, by only once heating the steel to the temperature above the A0 transformation point or to the temperature between A0 and AC3 transformation points with rapid heating rate and short holding time as already mentioned.

In the first hardening process the test steel is heated up to the temperature range between AC3 and 950 C., held at the temperature in that range for 1 to 3 seconds and quenched. Subsequently, for the second hardening process the steel is heated to the temperature in the transformation range of A0 and AC3 transformation points, kept and quenched under the same conditions as in the first hardening process. The second hardening process is followed by the tempering process, Which is carried out at temperature between A0 and 500 C. It is also possible to interpose a third hardening process between the second hardening process and the tempering process, which is carried out under the same conditions as in the second hardening process.

On the other hand, the inventors ascertained also that an improved notch-toughness is obtained by carrying out the hardening and tempering treatments under the same conditions as in the ordinary hardening and tempering treatments, that is, with a slow heating rate of about '30 to 40 minutes and a long holding time of about 60 minutes per inch in plate thickness maximum, provided that the hardening treatment is repeated as above mentioned. In this case the tempering process is carried out at temperature below the A0 transformation point.

To sum up, no particular improvement in the notchtoughness is obtained by carrying out only once the hardening and tempering process with slow heating rate and long holding time as in the ordinary hardening treatment, and a little improvement is obtained, if the heating rate is sped up and the holding time is shortened in the ordinary hardening process. However, a notable improvement in the notch-toughness is obtained when the hardening treatments are repeated once or more than once as in the present invention regardless of whether the heating rate is quick or slow or the holding time is short or long, though as previously mentioned where the heating time is slow there is the maximum in the holding time.

This new fact as discovered by the inventors may be explained as follows: By heating the steel consisting 4 Example 1 A process of a rapid heating rate and a short holding time has been applied to the specimens Nos. 1 and 3 of steels having the compositions as shown in the follow- 5 mainly of ferrite and pearhte to the temperature above mg table. The microscopic structure of these steels are the A0 transformation point the structure of the steel consisting mainly of fernte and pearlite.

TABLE I Percent Transformation temperature in Steel 0. material 0 st M11 P S Cr V A1 A01 A03 will be transformed to austenite, and this austenite will be reverted to the original ferrite and pearlite by quenching the heated steel. However the reverted ferritic and pearlitic grains are refined thereby to some degree. In the second hardening process, by reheating the hardened steel to the temperature in the transformation range of A0 and A0 pearlite will be transformed to austenite but ferrite will be only partially transformed to austenite. A part of ferrite remains as it is. By quenching the heated steel austenite formed by the second hardening process will be reverted again to ferrite and pearlite, thereby further refining the ferritic and pearlitic grains. The notch-toughness of the steel is remarkably improved and stabilized by the subsequent tempering treatment due to the refining of the ferritic and pearlitic grains resulting from the first and second hardening treatments and the remaining oversaturated ferritic structure which was not transformed to austenite in the second hardening process. The essential feature of the process of the present invention lies in that the steel mainly consisting of ferrite and pearlite is heated to the temperature above the A0 transformation point in the first hardening process and subsequently the second hardening treatment is carried out, in which the steel is reheated to the temperature in the transformation range of A0 and Ac The reason for reheating the steel hardened by the first hardening only to the temperature in the transformation range of Ac and Ac in the second hardening treatment is that if the heating to the temperature above the A0 transformation point were to be repeated in the second hardening treatment the oversaturated ferrite would not remain in the structure and thus would not largely contribute to remove the brittleness of the steel. Additionally, if the heating temperature in the second hardening treatment were to be below the A0 transformation point the sufiicient refining the ferritic and pearlitic grains would not be effected.

In regard to the starting material, it was found that no satisfactory results were obtained by the heat treatment of the present invention, when applied to such steels as will form substantially martensitic structure after the first hardening. This is presumably due to the martensite formed by the first hardening being completely decomposed into ferritic carbide. When the steel is subjected to the second hardening, that is, the steel is reheated to the temperature between A0 and AC3 transformation points this ferritic carbide structure will remain even after the tempering treatment has been carried out. Therefore, the heat treatment of the present invention may be effectively applied only to steel consisting of not less than 50% of ferrite and pearlite. The carbon content of such steels is in the range of 00.4%

According to the method of the present invention the specimen No. 1 steel (carbon steel, round bar 20 mm. in diameter) was heated from the room temperature up to 900 C. in about 2 minutes by means of the hardening apparatus by high-frequency current, then was held at this temperature for 1 to 3 seconds and thereupon quenched (the first hardening). Subsequently the material was subjected to the second hardening, in which it was heated from the room temperature up to 800 C. in about 2 minutes, held at this temperature for 1 to 3 seconds and quenched. The second hardening was followed by the tempering treatment, in which the material was heated up to 600 C. in one case and to 650 C. in another case and held for 60 minutes in both cases.

A further experiment was carried out, in which the third hardening treatment succeeded the second hardening treatment. In the third hardening treatment the material was heated from the room temperature up to 750 C. in about 2 minutes, then was held for 1 to 3 seconds and quenched. The third hardening was followed by the same tempering treatments as aforesaid.

On the other hand, for the sake of comparison, an ordinary hardening process was applied to the same specimen No. 1 in which a hardening treatment is to be carried out only one time. In this case the material was heated from the room temperature to 950 0., 900 C. and 750 C. in a heating furnace in to 40 minutes respectively, and quenched after being held for 60 minutes. The hardening treatment was followed by the tempering treatment in the same conditions as above mentioned.

The same treatments wer applied to the specimen No. 3 (low alloy steel, round bar 20 mm. in diameter).

The results of the above experiments on the test steels specimens Nos. 1 and 3 are shown in the following Table 2. According to the V-notch Charpy test on the results of the experiments obtained 'by the ordinary process, in which the hardening treatment was carried out only one time, the 50% shearing fracture transition temperature (vTrs), which indicates the notch-toughness, was shown in the range of 59 to 90 C. in the case of the specimen No. 1 and -12 to 43 C. in the case of the specimen No. 3.

On the contrary, when applying the processes according to the present invention, that is, carrying out the hardening treatments twice or three times in succession, the vTrs showed 96 to ll3 C. for the specimen No. 1 and 60 to 64 C. for the specimen No. 3, indicating a remarkable improvement in the notch-toughness, as compared with in the case of applying the ordinary hardening 5 treatment.

TABLE 2 First quick Second quick Third quick Steel heating and heating and heating and TempervE-120 vTrs uTr15 UEo Hardmate- Treatment quenching quenching quenching ing, C. (kgm./ 0.) C.) (kgmJ ness rial treatment, treatment, treatment, cm?) cm?) (H) 600 3. 0 64 -78 139 600 2. 5 -90 117 154 Ordinary process 650 3. 0 78 115 151 600 1, 0 84 100 142 1 650 1. 0 59 94 142 600 5. 1 '113 154 137 Process of the present invention 650 8. 9 100 134 137 600 0. 5 108 -138 133 650 4. 0 96 -'122 137 670 43 76 221 Ordinary process 228 g% :g? 38% 600 -14 62 107 3 650 12 -70 181 600 63 114 195 Process of the present invention..- 650 62 114 188 l 600 54 -106 101 650 60 112 181 1 Example 2 The specimens Nos. 2 and 4 as shown in Table 1 were treated according to the processes of the present invention with a slow heating rate and along holding time.

The specimen No. 2 (carbon steel, 20 mm. in plate thickness) was heated from the room temperature to 900 C. in 30 to 40 minutes in the heating furnace, held for 30 minutes at this temperature and then quenched (the first hardening process). The first treatment was followed by the second and third hardening treatments, in which the steel was heated from the room temperature to 800 or 840 C. in 30 to 40 minutes, held for 15 minutes at these temperatures and quenched respectively. The tempering process succeeded the second or the third hardening, in which the steel was heated to 500 or 600 C. and held for 40 minutes at these temperatures.

On the other hand, the same specimen was subjected to the ordinary hardening treatment, that is, was subjected to the heat treatment only one time, in which the steel was heated from the room temperature to 900 C in 30 to 40 minutes in the heating furnace, held for 30 to 40 minutes at this temperature and then quenched. The hardened steel was further subjected to the tempering treatment, in which it was heated to 600 or 630 C. with a holding time of 40 minutes.

The specimen No. 4 (low alloy steel, mm. in plate thickness) was also treated under the same conditions as for the specimen N0. 2.

The results of the experiments are shown in Table 3.

In comparing Tables 2 and 3 it has been established that in case of applying the processes of the present invention a remarkable improvement of the steel consisting mainly of ferrite and pearlite with a high notch-toughness may be obtained, regardless of the conditions of hardening treatment, that is, whether the heating rate is rash or slow, or the holding time is long or short. The only condition to be maintained is that a holding time in the second hardening treatment should not be unnecessarily long, otherwise the remarkable improvement in the notchtoughness can not be secured. The maximum holding time is to be limited to about 60 minutes per one inch in plate thickness.

Whether the tempering treatment is carried out immediately after the second or the third hardening treatment or after the lapse of a considerable time thereafter, the same mechanical properties of steel can be obtained.

What we claim is:

1. A process for heat-treating carbon steel consisting of at least ferrite and pearlite, which comprises the steps of heating said carbon steel to a temperature between the AC3 transformation point and 950 C. and quenching the heated steel, reheating the steel to a temperature between the transformation points Ac and AC3, holding it at this temperature for a maximum of minutes per inch of plate thickness, then quenching the heated steel and subsequently subjecting the treated steel to a tempering treatment in which the hardened steel is heated to a temperature below the Ac, transformation TABLE 3 Steel Yield Tensile Elonmate- Treatment First quenching Second quench- Third quench- Tampering point strength gation vTr15 vTrs rial treatment ing treatment ing treatment (kg. (kg./ (per- 0.) 0.)

mm?) mm?) cent) Kept at 900 C. Kept at 600 C. 39.7 48. 5 42. 1 -124 for 30 min. for 40 min. do Kept at 630 C. 39. 9 48. 8 43. 2 118 -90 Ordinary process for 40 min.

Kept at 900 C. Kept at 800 C. Kept at 760 C. Kept at 500 C. 41. 5 51. 4 37. 2 168 for 30 min. for 15 min. [or 15 min. for 40 min. 2 Process of the present do do Kept at 800 0. do 40. 9 50. 5 40. 4 162 117 invention. for 1-5 min.

d0 do Kept at 840 C. Kept at 600 C. 39. 5 48. 9 40. 8 141 112 i for 15 min. for 40 min. d0 Kept at 840 C. Kept at 800 C. (10 38. 8 48. 9 41. 8 169 119 for 15 min. for 15 min. do d0 Kept at 840 C. (10 40. 0 49.9 41. 4 171 134 for 15 min. Ordinary process Kept at 900 C. Kept at 600 C. 57. 4 65. 5 27. 9 50 24 for 30 min. for 40 mm. Kept at 900 C. Kept at 840 C. Kept at 600 C. 60. 8 69 2 26. 8 71 -47 4 Process of the present for 30 min. for 15 min. for 40 min.

invention. -do .do Kept at 840 0. d0 51. 7 62. 5 32. 8 78 52 for 15 min.

As seen from Table 3, also in the case of applying the hardening treatment of a slow heating rate and a long holding time a remarkable improvement in the notchtoughness of the steel may be obtained so far as the processes of the present invention are applied.

point, thereby obtaining carbon steel having good notchtoughness.

2. A process for heat-treating carbon steel consisting of at least 50% ferrite and pearlite, which comprises (1) heating said carbon steel to a temperature between the A transformation point and 950C. and quenching the heated steel, (2) reheating the hardened steel to a temperature between the transformation points Ac and A0 holding it at this temperature for a maximum of 60 minutes per inch of plate thickness, and then quenching said steel, (3) reheating the hardened steel to a temperature between the transformation points AC1 and A0 holding it at this temperature for a maximum of 60 minutes per inch of plate thickness, and then quenching said steel, and (4) subjecting the treated steel to a tempering treatment, in which the said steel is heated to a temperature below the Ac transformation point,'thereby obtaining carbon steel having good notch-toughness.

3. A process for heat-treatment of steels claimed in claim 1 wherein the steel is a low alloy steel consisting of at least 50% ferrite and pearlite.

4. A process for heat-treating carbon steel consisting of at least 50% ferrite and pearlite, which comprises the steps of heating said carbon steel to a temperature above the A0 transformation point and quenching the heated steel, reheating the steel to a temperature between the transformation points A0 and A0 holding it at this temperature for a maximum of 60 minutes per inch of plate thickness, then quenching the heated steel and subsequent- 5. A process for heat-treating carbon steel consisting of at least ferrite and pearlite, which comprises (1) heating said carbon steel to a temperature above the A0 transformation point and quenching the heated steel, (2) reheating the hardened steel to a temperature between the transformation points Ac and AC3, holding it at this temperature for a maximum of minutes per inch of plate thickness, and then quenching said steel, (3) reheating the hardened steel to a temperature between the transformation points A0 and A0 holding it at this temperature for a maximum of 60 minutes per inch of plate thickness, and then quenching said steel, and (4) subjecting the treated steel to a tempering treatment, in which the said steel is heated to a temperature below the A0 transformation point, thereby obtaining carbon steel having good notch-toughness.

References Cited by the Examiner UNITED STATES PATENTS 3,028,270 4/1962 Morita et al. 148143 3,110,635 11/19 63 Gulya 148-36 3,178,324 4/1965 Grange et al 148l43 DAVID L. RECK, Primary Examiner.

HYLAND BIZOT, Examiner.

C. N. LOVELL, Assistant Examiner. 

2. A PROCESS FOR HEAT-TREATING CARBON STEEL CONSISTING OF AT LEAST 50% FERRITE AND PEARLITE, WHICH COMPRISES (1) HEATING SAID CARBON STEEL TO A TEMPERATURE BETWEEN THE AC3 TRANSFORMATION POINT AND 950*C. AND QUENCHING THE HEATED STEEL, (2) REHEATING THE MARDENED STEEL TO A TEMPERATURE BETWEEN THE TRANSFORMATION POINTS AC1 AND AC3, HOLDING IT AT THIS TEMPERATURE FOR A MAXIMUM OF 60 MINUTES PER INCH OF PLATE THICKNESS, AND THEN QUENCHING SAID STEEL, (3) REHEATING THE HARDENED STEEL TO A TEMPERATURE BETWEEN THE TRANSFORMATION POINTS AC1 AND AC3, HOLDING IT AT THIS TEMPERATURE FOR A MAXIMUM OF 60 MINUTES PER INCH OF PLATE THICKNESS, AND THEN QUENCHING SAID STEEL, AND (4) SUBJECTING THE TREATED STEEL TO A TEMPERING TREATMENT, IN WHICH THE SAID STEEL IS HEATED TO A TEMPERATURE BELOW THE AC1 TRANSFORMATION POINT, THEREBY OBTAINING CARBON STEEL HAVING GOOD NOTCH-TOUGHNESS. 